Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures. Some of these systems use magnetic field measurements. For example, U.S. Pat. Nos. 5,391,199 and 5,443,489, whose disclosures are incorporated herein by reference, describe systems in which the coordinates of an intrabody probe are determined using one or more field transducers. Such systems are used for generating location information regarding a medical probe, such as a catheter. A position sensor is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field generators, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations.
Additional methods and systems that relate to magnetic position tracking are described, for example, in PCT Patent Publication WO 96/05768, U.S. Pat. Nos. 4,849,692, 4,945,305, 5,453,686, 6,239,724, 6,332,089, 6,618,612 and 6,690,963 and U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1, 2004/0068178 A1 and 2004/0147920 A1, whose disclosures are all incorporated herein by reference. These publications describe methods and systems that track the position of intrabody objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures.
Some position tracking systems, including some of the systems described in the above-mentioned references, use alternating-current (AC) magnetic fields. Other position tracking systems, such as the systems described in U.S. Pat. Nos. 4,849,692, 4,945,305 and 5,453,686 cited above use direct-current (DC) fields.
Several position sensors and sensor assemblies for sensing magnetic fields are known in the art. For example, U.S. Pat. No. 6,536,123, whose disclosure is incorporated herein by reference, describes a hybrid three-axis magnetic sensor for calculating the direction of the earth magnetism. The sensor includes a flux gate type magnetic sensor which is so formed that a base serves as a main member and detects two axis components of a magnetic vector defined by a plane parallel to the base. A Hall element detects another component of the magnetic vector orthogonal to the base. A tilt sensor detects a tilt angle of the base. The flux gate type magnetic sensor and the Hall element are integrally structured together as a hybrid IC. The detected three-dimensional magnetic vector is corrected in light of the inclination of the base.
As another example, U.S. Pat. No. 6,278,271, whose disclosure is incorporated herein by reference, describes a magnetic field sensor for measurement of the three components of a magnetic field. The sensor comprises a Hall-effect element and an electronic circuit. The Hall-effect element comprises an active area, which is contacted with voltage and current contacts. Four voltage contacts are connected to inputs of the electronic circuit. By means of summation or differential formation of the electrical potentials present at the voltage contacts, the electronic circuit derives three signals proportional to the three components of the magnetic field.
U.S. Pat. No. 6,184,680, whose disclosure is incorporated herein by reference, describes a magnetic field sensor in which a magnetic film or films having a magneto-resistance effect for detecting a magnetic field and a conductor electrode film for applying a current to the magnetic film are deposited on a flexible substrate.
Magnetic field sensors sometimes comprise magneto-resistive sensors. For example, several magnetic field sensors and modules based on magneto-resistive elements are produced by Honeywell International Inc. (Morristown, N.J.).
Information regarding these products can be found at www.ssec.honeywell.com/magnetic/products.html. Philips Electronics (Amsterdam, The Netherlands) also produces magneto-resistive field sensors. Details regarding these products can be found at www.semiconductors.philips.com.